Superconductors are materials which have no resistance to flow of electricity. Some applications of superconductive materials are power transmission lines, electric motors and generators, current leads, nuclear magnetic resonance analyzers, and magnets for use in levitated vehicles, energy storage systems, particle accelerators, and magnetic separation of materials. This invention deals with Bi--Sr--Ca--Cu--O (BSCCO) superconductors. One of the superconducting phases of the BSCCO system is Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.y. This phase is commonly termed Bi-2212, referring to the ideal stoichiometry, though this superconducting phase actually has a range of compositions. Bi-2212 may be viewed as a region on a three-dimensional phase diagram of the BSCCO system. Further information may be found in T.G. Holesinger et al., "Solid Solution Region of the Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.y Superconductor", Physica C 217 (1993) pp. 85-96.
Melt processing may be utilized in producing superconductive articles of Bi-2212 from Bi-2212 powders or powders comprised of Bi-2212 precursors. A powder is combined with a carrier, such as a silver tube, a substrate, or a container and subjected to melt processing and other operations to form a useful superconductive article. One method of forming such an article is termed the powder-in-tube (PIT) process. A tube of silver or silver alloy is filled with powder, sealed at the ends, mechanically deformed, and then subjected to melt processing. Mechanical deformation involves drawing through dies to form a wire and/or passing through rollers to form a tape. In another method of forming a superconductive article, a powder may be mixed with a liquid and disposed on a substrate to form a thick or thin film and the resulting assembly subjected to melt processing conditions to form the article. Another method of forming a superconductive article from powder is to place the powder in a container and melt process it. The resulting solid body may be removed from the container or the container may remain a part of the superconductive article.
Melt processing of Bi-2212 superconductive powder or precursor powder is accomplished by subjecting the powder or an assembly comprised of the powder to high temperatures and holding at those temperatures, preferably in a controlled atmosphere. The powder is partially melted and the assembly is cooled in such manner as to form a crystalline solid having superconductive phases. A problem which arises in melt processing of a Bi-2212 superconductor is incongruent melting, that is, the powder transforms into liquid phases and crystalline phases. Then, upon cooling, it is difficult to cause the material to transform into the superconducting phase. A further problem is the phase segregation which occurs upon melting. Several of the solid phases present when Bi-2122 is partially melted are less dense than the liquid phase and, therefore, migrate to the top portion of the partial melt. Such phase segregation makes it difficult to form a large amount of the superconducting phase upon cooling. Both of these problems adversely affect the superconducting properties of Bi-2212.